Positron emission tomography (PET) does not measure three-dimensional images of an observed object or subject directly. Instead, a small dose of a positron emitter within the object or subject is brought into the field of view (FOV) of the scanner. Positrons that are generated during a scan can only travel a few millimeters until they annihilate with an electron of the surrounding matter. In this process, a pair of simultaneous gamma particles each with an energy of 511 keV is generated, which gamma particles travel nearly perfectly in opposite directions. The gamma particles can be stopped and detected by means of a large number of small scintillator crystals. The PET scanner accumulates the detected coincident light events between all possible crystal pairs into “line of response” or LOR bins. The three-dimensional distribution of the positron emitter within the FOV can then be reconstructed from the accumulated LORs by a suitable reconstruction algorithm.
Prior to such reconstruction, a series of pre-processing steps has to be performed, including a “normalization” step which corrects for variations in the counting efficiencies of different crystal pairs. This normalization procedure removes artifacts from the reconstructed images and results in a much smoother appearance of the images. To determine the correction factors, it is necessary to measure a phantom that uniformly fills the PET scanner field of view with radioactivity. The normalization or correction factors are then the ratio between expected and actually measured count rates.
Additionally, a phantom may be used for daily quality control measurements of the scanning machinery. By imaging the phantom with its known geometry and radiation distribution, the accuracy of the software used to assemble the various tomographic slices acquired by the imaging apparatus into three-dimensional representations of a patient's region of interest can be assessed and, if necessary, the various apparatus settings can be adjusted.
Optimally or ideally, a normalization phantom should exhibit the same amount of radioactivity along every possible pathway (i.e., LOR) between two crystals. In practice or realistically, however, a phantom may be used so long as it satisfies the following three criteria:                a) the coefficients of scattering or absorption of 511 keV gamma rays generated within the phantom should be as small as possible;        b) every monitored LOR of the scanner must intersect with the radioactive region of the phantom (which includes typically all LORs that intersect with the field of view of the PET scanner); and        c) the geometry and positioning of the phantom have to be known exactly, such that software can correct for the non-uniformity of the amount of radio-activity between the detector crystal pairs.        
Usually, phantoms with uniform distribution of a radioactive substance are filled with solid plastic materials that can be produced by a curing process of one or more liquids into which a radioactive substance is injected while the material is still liquid. If the materials are mixed properly, the radioactivity is perfectly uniformly distributed throughout the phantom. Unfortunately, however, the attenuation length of the commonly used plastic materials is very short. For example, typical polyethylene-based plastics with a density of around 1.1 g/cc have an attenuation length of about 9 to 10 cm and a Compton scattering fraction of nearly 100 percent. Therefore, it is not possible to build larger phantoms, which could flood the entire field of view, that do not suffer from unacceptably high absorption/scattering.
To compensate for that limitation, it is known to sweep one or more smaller phantoms through the field of view during the normalization scan. For example, two cylindrical phantom rods may be fixed parallel to each other and rotated or orbited by means of an electric motor about the parallel line (axis) that extends between the two of them. Averaged over time, the sweep generates a cylindrical “net” or overall phantom that fills the field of view yet that exhibits no or minimal scattering or absorption of 511 keV gamma rays.
Recently, however, an integrated magnetic resonance/PET (or MR/PET) scanner has been developed (see, e.g., U.S. Pub. No. 2007/0055127, published Mar. 8, 2007 and incorporated herein by reference), and ordinary electric motors do not operate properly within the strong magnetic field produced by the MR components of the apparatus. Accordingly, the current state of the art teaches that the PET components of the integrated apparatus must be separated from the MR components of the apparatus when the phantom-based normalization data set is being acquired. Such protocol, however, is inconvenient as well as time- and space-consuming. Moreover, it carries with it the risk that the various apparatus setup parameters may be changed during the disassembly and reassembly process.